The present invention relates to a process for forming a refractory metal silicide layer. It more particularly applies to refractory metals having a melting point above 1400.degree. C., e.g. metals of groups IVa, Va, VIa of the periodic classification of elements, such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. More specifically, it relates to the production of interconnection layers for the manufacture of integrated circuits.
In the field of microelectronics and more particularly that of silicon integrated circuits using bipolar or MOS transistors, it is necessary to interconnect the elementary devices, while also connecting them to other components of the circuit and external supply means, this being carried out by means of conductive films called interconnection layers.
Hitherto, these interconnection layers have been made from aluminum, which is highly conductive, but suffers from the disadvantage of reacting with silicon, even when the latter is in SiO.sub.2 form for forming aluminum silicon alloys.
In connection with the miniaturization of circuits, the lateral sizes of components have been reduced to permit the placing of a larger number of components on the same circuit. Therefore the thickness of these various films has decreased and the problem of the interconnection layers reacting with the silicon has become more serious.
In order to prevent these interactions, there have been inserted between the aluminum and the compounds containing silicon layers acting as diffusion barriers and which are, e.g., made from chromium or tungsten.
In order to bring about a better alignment of the source and drain of a MOS transistor, use has also been made of a highly doped polycrystalline silicon gates for bringing about the self-alignment. Thus, aluminum has been progressively replaced by highly doped polycrystalline silicon, but this silicon suffers from the disadvantage of not being sufficiently conductive.
Thus, present research is directed towards the use of new materials, such as refractory metal silicides, for producing interconnection layers and for shunting the gate polycrystalline silicon. Thus, silicides of refractory metals are of interest because they are stable at the high temperatures used during the heat treatments involved in the production of integrated circuits and they have a high electrical conductivity. These silicides can be formed at high temperature by thermal growth from a metal layer deposited on polycrystalline silicon However the use of high temperatures is prejudicial, because they modify the physicochemical and electronic characteristics of the other layers and components already formed on the integrated circuit.
It is also possible to form such silicide films by the co-evaporation of two elements, e.g. silicon and tungsten, under a high vacuum (10.sup.-7 Torr), followed by an annealing treatment at 1000.degree. C. under a hydrogen atmosphere, as described in Appl. Phys. Lett, 33(1), 1978, pp 76-78. However, this process suffers because it does not obtain a yield high enough for the deposition.
It is also possible to form such films by subjecting a refractory metal film deposited under an ultravacuum on a silicon substrate to hot ionic implantation of As.sup.+, followed by an annealing treatment, in the manner described in Appl. Phys. Lett, 37(1), 1980, pp. 295-298. However, this process also suffers from the disadvantage of requiring high vacuums. In addition, it is necessary to have high implantation currents (1.5 .mu.A.cm.sup.-2) and it is necessary to carry out simultaneous heating (350.degree. C.) to obtain the silicide layer.
Attempts at manufacturing refractory silicides at lower temperatures have not as yet been made due to the fact that the silicide formation is blocked by the presence of oxygen traces, as indicated by D. J. Silversmith, D. D. Rathman and R. W. Mountain in "Thin Solid Films" Vol. 93, p. 413, 1982.